After a well has been drilled to a subsurface production zone which is found to contain paying quantities of petroleum products such as natural gas, oil, distillate, etc., the well is completed by extending production tubing through the casing lining the well bore to the level of the production zone. Typically, packers are set in the casing and between the casing and tubing to isolate the production zone and to ensure that pressurized fluid from the production zone can pass only through the production tubing to a wellhead control system located at the surface. To prepare the production zone for optimum production of petroleum products, it is almost always desirable to circulate washing fluid into the well which frequently contains chemical constituents such as acid, for example, for chemical treatment of the production zone. After well completion has been accomplished and production has continued for an extended period of time, it is sometimes appropriate to circulate fluid into the well for the purpose of enhancing production, cleaning the tubing or providing other desirable well treating characteristics. Various acids and chemicals are typically employed for such purpose.
At times during the life of a well, it is necessary to temporarily kill the well to permit certain servicing operations or to permit repair or replacement of production equipment. In some cases, emergency conditions arise where it becomes necessary to shut the well in and kill it within a relatively short period of time.
Wells are typically killed by pumping kill fluid of various suitable liquid forms into the well to the extent that the pressure of the formation is overcome by the hydrostatic head of the column of kill fluid standing above the formation. A small amount of kill fluid may enter the formation and function to develop a seal that isolates the pressure of the formation from the well bore. After the well has been killed, production equipment such as packers, production tubing and other equipment may be safely removed from the casing for repair or replacement. After the production equipment has been reinstalled in the well casing, production is re-established by circulating a cleaning medium into the well which removes the kill fluid and re-establishes fluid communication between the production formation and the casing. It is difficult to determine just how much kill fluid must be pumped into a well to safely kill the well. In many cases, sufficient amounts of kill fluid are pumped into the formation that the formation can be permanently damaged to the extent that adequate production from the formation cannot be re-established. It is, of course, desirable to provide a system for safely killing the well and which also ensures against the likelihood of causing damage to the production formation by excessive kill fluid.
At times, it is desirable to kill a well without allowing kill fluid to come into contact with the producing formation. In this case, the tubing and packer are allowed to remain in place while repair services are conducted.